You are definitely not too old. I was in the Navy, and in basic training I was 19 and was finding it hard to keep up with the older people.
From what I noticed, it also seemed as if the older people handled the tough treatment better as well. They always kept their cool and did not fly off the handle, where some of the 18 year olds tried to act big and talk back.
Get with your recruiter and find out what the physical requirements are, and start working on them. I dont know about the marines, but in the navy you had the option to go in at a higher rank if you took a written and physical test before you shipped out. You may want to look into that as well.
Good luck!
